In RGBA mode,
pixels can be drawn using a function that blends the incoming (source)
RGBA values with the RGBA values that are already in the frame buffer (the
destination values). Blending is initially disabled. Use glEnable and glDisable
with argument GL_BLEND to enable and disable blending.

glBlendFunc defines
the operation of blending when it is enabled. sfactor specifies which of
nine methods is used to scale the source color components. dfactor specifies
which of eight methods is used to scale the destination color components.
The eleven possible methods are described in the following table. Each method
defines four scale factors, one each for red, green, blue, and alpha.

blending arithmetic is not
exactly specified, because blending operates with imprecise integer color
values. However, a blend factor that should be equal to 1 is guaranteed
not to modify its multiplicand, and a blend factor equal to 0 reduces its
multiplicand to 0. For example, when sfactor is GL_SRC_ALPHA, dfactor is
GL_ONE_MINUS_SRC_ALPHA, and $A sub s$ is equal to $k sub A$, the equations
reduce to simple replacement:

(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) with primitives
sorted from farthest to nearest. Note that this transparency calculation
does not require the presence of alpha bitplanes in the frame buffer.

Blend
function (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) is also useful for rendering
antialiased points and lines in arbitrary order.

Polygon antialiasing is
optimized using blend function
(GL_SRC_ALPHA_SATURATE, GL_ONE) with polygons sorted from nearest to farthest.
(See the glEnable, glDisable reference page and the GL_POLYGON_SMOOTH
argument for information on polygon antialiasing.) Destination alpha bitplanes,
which must be present for this blend function to operate correctly, store
the accumulated coverage.